Numerous examples are available in the art for the production of an olefin polymerization catalyst by combining a solid component containing at least magnesium, titanium and chlorine with an activating organoaluminum compound. These catalysts are conventionally referred to as supported coordination catalysts or catalyst systems. The activity and stereo-specific selectively of such catalysts is generally improved by incorporating an electron donor, i.e., a Lewis base, in the solid component and by employing as a third catalyst component an electron donor which is complexed in whole or in part with the activating organoaluminum compound. Employing terminology which is conventional in the art of polymerization catalysts, the solid titanium-containing compound is referred to as a "procatalyst," the organoaluminum compound, whether complexed or not, is referred to as the "co-catalyst" and the third component electron donor, whether used separately or partially or totally complexed with the organoaluminum compound is referred to as a "selectivity control agent" (SCA).
Supported coordination catalysts of this type are broadly and generally known to produce olefin polymers in high yield and, in the case of polymers of propylene or higher .alpha.-olefins, to produce polymers with a high selectivity to stereoregular polymers. The activity of the coordination catalyst is of particular importance because of the desirability of avoiding the necessity to extract catalyst residues from the polymer in a de-ashing step. The selectivity to stereoregular polymer is of importance because of the desirability of avoiding a process step to extract atactic polymer when .alpha.-olefins of three or more carbon atoms are polymerized.
Many chemical combinations of procatalysts, co-catalysts and selectivity control agents are known in the art to produce active catalysts. However, considerable experience has resulted in certain materials being of greater interest. The procatalyst is typically some chemical combination of magnesium chloride and titanium tetrachloride and the electron donor is frequently an aromatic ester such as ethyl benzoate or ethyl p-toluate. The co-catalyst is frequently an aluminum trialkyl such as triethylaluminum or triisobutylaluminum which is often complexed with a portion of the selectivity control agent which is also, typically, an aromatic ester. While variations in any of these catalyst components will influence the performance of the catalyst which results, the component which seems to offer the greatest opportunity for modification to produce greater catalyst activity is the procatalyst.
Preferred methods of producing procatalysts are the subject of a number of U.S. patents including U.S. Pat. Nos. 4,329,253, 4,393,182, 4,400,302 and 4,414,132. These procatalysts, useful in the production of olefin polymerization catalysts of high activity and stereoregular selectivity, are generally prepared by contacting a magnesium compound, titanium tetrachloride and the electron donor in the presence of a halohydrocarbon. The resulting particles are then contacted with additional titanium tetrachloride before washing off the excess titanium tetrachloride with a light hydrocarbon solvent, e.g., isooctane, and drying the procatalyst.
Even in instances where an olefin polymerization catalyst composition of acceptable catalytic activity and stereospecific selectivity is obtained, other considerations are important. It is highly desirable to produce polyolefin polymer having a specific morphology, e.g., polymer product in the physical form of spheroidal particles of uniform size and without the presence of any substantial quantity of small, irregular polymer particles such as fines. It is known that in olefin polymerizations the morphology of the polymer product is a replica of the morphology of the catalyst employed in its production. It is also known that the morphology of the catalyst is dictated to a considerable extent by the morphology of the procatalyst and particularly by the morphology of the magnesium compound used as the support. Thus, production and use of a magnesium compound of desirable morphology will generally result in the ultimate production of polyolefin polymer of desirable morphology.
A variety of procedures are available which are designed to produce a magnesium compound support, or the transition metal catalyst produced therefrom, of improved morphology. GB No. 2,101,610 reacts a solid particulate material with an organic magnesium compound, treats the resulting magnesium compound with oxygen, carbon dioxide or a hydroxyl compound and reacts the treated product, simultaneously or in sequence, with a carbonyl compound and a transition metal compound. In another process a dispersion of solid magnesium alkoxide particles in a suitable liquid medium are employed as the feed for a spray drying process, but the solid particles are a disadvantage because of the tendency of the particles to clog the fine orifices of the spray drying equipment. In U.S. Pat. No. 4,540,679 a magnesium compound, i.e., a magnesium hydrocarbyl carbonate, is used as the support which is produced by reacting a suspension of magnesium alkoxide and carbon dioxide. In U.S. Pat. No. 4,771,024 a solution of a carbonated magnesium alkoxide is produced by bubbling carbon dioxide through a suspension of magnesium ethoxide in a diluent such as an alkanol. The resulting solution is used to impregnate a solid inorganic support or to produce particulate magnesium ethoxide by spray drying. The support particles thereby obtained are of desirable morphology and are useful precursors of olefin polymerization procatalysts. However, some degree of care must be taken during the reaction of the magnesium alkoxide and the carbon dioxide because of the exothermic nature of the reaction and the viscous nature of the solution which results.